The present invention disclosed herein relates to a field emission X-ray tube and a method of operating the same, and more particularly, to a subminiaturized field emission X-ray tube and a method of operating the same.
A typical field emission X-ray tube generates electrons from a field emission emitter by a mesh-type gate electrode provided in a vacuum vessel, accelerates the generated electrons to several to several tens of kV, and makes the electrons hit a target anode electrode to thereby generate an X-ray. The generated X-ray is constituted of a characteristics X-ray determined by unique characteristics of materials used for the target anode electrode and a continuous X-ray generated due to deceleration of the accelerated electrons. Herein, one or more focusing electrodes are selectively added between an anode electrode and a gate electrode so that an electron beam is focused to one point of the anode electrode.
Since the mesh-type gate electrode, where a hole is formed for the electron beam to pass, is relatively close to a cathode electrode in comparison with the anode electrode, an insulator is inserted to the mesh-type gate electrode for maintaining electric insulation. Further, an insulator is provided between the focusing electrode and the gate electrode for electric insulation between them. These insulators include nonconductive materials such as glass and ceramic, and may be formed in various shapes according to use of an X-ray tube. As described above, the insulators are layered between the gate electrode and the focusing electrodes causing complicated structure. Therefore, subminiaturization and manufacturing process are difficult.
For operating a field emission X-ray tube, a positive gate voltage is applied to the gate electrode, and a positive acceleration voltage is applied to the anode electrode based on electric potential of the cathode electrode. Herein, a voltage for focusing the electric beam is applied to the focusing electrode, and the voltage applied to the focusing electrode may be changed according to operation conditions. According to such an operation method, the gate voltage should be applied as a pulse form for generating a pulse-type X-ray. Further, in the case of applying a high gate voltage, a driving circuit is complicated for applying a voltage ranging from several volts to hundreds of volts as the pulse form. For instance, in the case that an operating filed of the field emission emitter is about 5V/μm, and a distance between the gate electrode and the cathode electrode is about 100 μm, the operating voltage applied to the gate electrode should be a pulse voltage of about 500V.
For protecting the field emission emitter from a electric field generated by a high voltage of the anode electrode, it is needed to form a diameter of the hole of the mesh-type gate electrode equal to or smaller than the distance between the gate electrode and the cathode electrode. In the case that the hole diameter of the gate electrode is larger than the distance between the gate electrode and the cathode electrode, the electric field generated by the anode electrode may influence the field emission emitter hindering 3-electrode operation of the field emission X-ray tube, or the field emission emitter may be damaged due to generation of an arc or the like.
Also, when electrons hit the target anode electrode and energy of the electrons are converted to thereby generate X-ray, most of electron energy is generated as heat due to low conversion efficiency. Therefore, cooling the anode electrode is an important factor for operating the field emission X-ray tube. In the case of the typical field emission X-ray tube, if a voltage of tens of kV is applied to the anode electrode, it is not easy to cool the anode electrode due to a limitation of insulation.
Moreover, in the case of using the mesh-type gate electrode, the field emission X-ray tube may be fabricated to have a diameter of several centimeters. However, if the field emission X-ray tube is subminiaturized to have a small diameter of several millimeters, it is not easy to fabricate the gate electrode and the focusing electrode.